2.1 WOUND HEALING
Currently available wound healing therapies involve the administration of therapeutic proteins. Such therapeutic proteins may include regulatory factors involved in the normal healing process such as systemic hormones, cytokines, growth factors and other proteins that regulate proliferation and differentiation of cells. Growth factors, cytokines and hormones reported to have such wound healing capacity include, for example, the transforming growth factor-.beta. superfamily (TGF-.beta.) of proteins (Cox, D. A., 1995, Cell Biology International, 19:357-371) acidic fibroblast growth factor (FGF) (Slavin, J., 1995, Cell Biology International, 19:431-444), macrophage-colony stimulating factor (M-CSF) and calcium regulatory agents such as parathyroid hormone (PTH).
A number of problems are associated with the use of therapeutic proteins, i.e. cytokines, in wound healing therapies. First, the purification and/or recombinant production of therapeutic proteins is often an expensive and time-consuming process. Despite best efforts, however, purified protein preparations are often unstable making storage and use cumbersome, and protein instability can lead to unexpected inflammatory reactions (to protein breakdown products) that are toxic to the host.
Second, systemic delivery of therapeutic proteins, i.e. cytokines, can be associated with serious unwanted side effects in unwounded tissue. Due to inefficient delivery to specific cells and tissues in the body, administration of high doses of protein are required to ensure that sufficient amounts of the protein reach the appropriate tissue target. Because of the short half life in the body due to proteolytic degradation, the proteins must also be administered repeatedly which may give rise to an immune reaction to the therapeutic proteins. The circulation of high doses of therapeutic proteins is often toxic due to pleiotropic effects of the administered protein, and may give rise to serious side effects.
Third, exogenous delivery of recombinant proteins is inefficient. Attempts have been made to limit the administration of high levels of protein through immobilization of therapeutic protein at the target site. However, this therapeutic approach complicates the readministration of the protein for repeated dosing.
Fourth, for a variety of proteins such as membrane receptors, transcription factors and intracellular binding proteins, biological activity is dependant on correct expression and localization in the cell. For many proteins, correct cellular localization occurs as the protein is post-translationally modified inside the cells. Therefore, such proteins cannot be administered exogenously in such a way as to be taken up and properly localized inside the cell.
As these problems attest, current recombinant protein therapies for wound healing are flawed, because they do not present a rational method for delivery of exogenous proteins. These proteins, i.e. cytokines, are normally produced at their site of action in physiological amounts and efficiently delivered to cell surface signaling receptors.